1. Field of the Invention
The present invention relates to a water base ink for ink-jet recording to be used for an ink-jet recording apparatus, and a method for preparing the water base ink for ink-jet recording.
2. Description of the Related Art
The printer technique based on the ink-jet recording system is a printing technique based on, for example, the bubble method in which the ink is discharged from minute nozzles by using bubbles generated by the rapid heating, or the piezoelectric method in which the ink is discharged from minute nozzles by using a piezoelectric element that is deformable when a voltage is applied. In this technique, the inks of several colors, which serve as the basic colors, are converted into minute liquid droplets of several picoliters to several tens picoliters, and they are selectively landed on the paper surface to form an image thereby.
Those known as the water base ink for ink-jet recording to be used for the ink-jet recording system as described above include those in which a water-soluble dye or a water-insoluble particulate coloring agent is dissolved or dispersed in a liquid medium composed of water and a water-soluble organic solvent. In recent years, it is demanded that a high printing quality, in which the contour of the image is sharp, is realized on many types of paper. Therefore, a water base ink for ink-jet recording is almost dominantly adopted, which is based on the use of the water-insoluble particulate coloring agent such as the pigment to successfully reduce the blurring owing to the coloring agent component remaining on the paper surface. In particular, as for the water-insoluble particulate coloring agent, those of the self-dispersing type, in which a surface treatment is applied so that the coloring agent is capable of being dispersed in water, are dominantly adopted.
A preparation method, which is called “letdown”, is generally adopted as a method for preparing the water base ink for ink-jet recording obtained by dispersing the water-insoluble particulate coloring agent as the coloring agent as described above. In this method, a preparation liquid is manufactured such that an aqueous coloring agent solution obtained by dispersing the water-insoluble particulate coloring agent is diluted with water so that the concentration of the water-insoluble particulate coloring agent is suitable for a desired coloring density. The other constitutive materials for the ink are added at appropriate timings to the preparation liquid while gently or slowly agitating the preparation liquid. According to the letdown method, it is possible to prepare the ink while maintaining the dispersion stability of the particulate coloring agent.
The water base ink for ink-jet recording is required to have the performance characteristics which are most appropriate to be used for the ink-jet recording system, in addition to the successful achievement of the high printing quality in which the contour of the image is sharp. For example, the following features are required. That is, there is no deterioration of the printing quality (hereinafter referred to as “bleeding” as well) which is caused by the mixing of the inks with each other at portions (hereinafter referred to as “boundary portions” as well) at which the inks having different colors are disposed adjacently when an image is formed on recording paper. The ink is dried quickly when an image is formed on recording paper, and the rub resistance is satisfactory. The ink is stable without causing any change of, for example, the characteristics and the physical properties of the ink over a long period of time. The ink does not corrode resin materials and metal materials which are used for the ink-jet head, and the stable jetting operation can be performed over a long period of time without causing any clog-up at the nozzle. In order to satisfy the performance characteristics as described above, a surfactant has been hitherto added to the ink for ink-jet recording. For example, Japanese Patent Application Laid-open No. 11-263930 discloses an ink for ink-jet recording based on the use of an amine oxide type surfactant in order that the recording on recording paper is performed at a uniform density and any uneven printing is reduced.
However, when the surfactant is added to the ink for ink-jet recording based on the use of the water-insoluble particulate coloring agent, the following problem has arisen. That is, when the ink is stored for a long period of time, then the surfactant is adsorbed to the water-insoluble particulate coloring agent, and the effect to reduce, for example, the uneven printing is declined or lost.